Transistors fabricated from semiconductor materials have a phenomenal track record in the relatively short history of microelectronics technology development. Although initial work on solid state transistors in the early 1950s focused on radar proximity fuse applications, this motivation gave way quickly to their use in digital electronics. Silicon-based metal oxide semiconductor field-effect transistor (MOSFET) technology is now a $300 billion per year business that enables affordable consumer adoption of life-changing information technology products (e.g., cell phones, computers, the Internet). At their most basic principle of operation, transistors control the flow of electrical current. Silicon complementary metal oxide semiconductor (CMOS) technology works by allowing the flow of small amounts of current pulses that represent binary data. MOSFETs, however, are not suitable for high-power microwave and high-power electronics applications because of low channel electron mobility and low breakdown voltages. Extending transistor operation to allow the flow of large amounts of current for high-power applications requires a different semiconductor materials technology. Recently, a new semiconductor material, gallium nitride (GaN), has emerged as an option for high-power applications.
GaN transistors are currently made from thin films of GaN material grown on substrates such as sapphire, silicon, or silicon carbide (SiC). High-electron-mobility transistors (HEMTs) made from Al1-xGaxN/GaN heterostructures are grown primarily by metal organic chemical vapor deposition (MOCVD) crystal growth techniques. Transistor structures with power added efficiencies of greater than 50% in microwave power amplifier applications have been demonstrated with all three growth substrate materials. However, SiC has higher thermal conductivity than sapphire and silicon. GaN HEMT on SiC technology is sufficiently developed so that commercial suppliers are now providing power amplifiers with x-band radio frequency (RF) power outputs in the range of 10 watts, which is sufficient for micro-radar applications. The pricing for these GaN HEMT products, however, is much higher than for equivalent GaN-based devices grown on sapphire or silicon due to the significantly higher costs for SiC growth substrates. Moreover, GaN growth on SiC requires a thin layer of aluminum nitride (AlN) for good epitaxy. Although GaN HEMTs on SiC growth substrates exhibit increased high-power performance as compared to devices fabricated on sapphire or silicon, a thermal boundary layer is associated with the thin AlN buffer layer material. The effective thermal conductivity of this layer can be as low as 2.2 watts per meter Kelvin (W/mK), which is more than 160 times smaller than the 360 W/mK thermal conductivity of SiC. Even though the AlN nucleation layer is thin, about 40 nanometers (nm), it contributes to significant additional heating of the GaN transistor material under high current operating conditions. Thin layers of such materials are known to cause large reductions in cross-plane thermal conductivity values due to phonon scattering or phonon reduction effects. Since GaN growth on SiC requires a thin AlN layer for good epitaxy, this thermal boundary property of the AlN layer stands as a major problem for the commercial viability of such GaN HEMTs fabricated with SiC growth substrates.
Silicon-based insulated gate bipolar junction transistors (IGBTs), a more than 20-year-old technology, are currently the incumbent technology for most high-power electronics applications. For example, hybrid and plug-in electric vehicles rely on Si IGBT modules for direct current (DC)-to-DC boost conversion and DC-to-alternating current (AC) inversion for driving electric traction motors. The relatively small bandgap energy of silicon limits the boosted voltage range to 600 volts or less. By contrast, larger bandgap GaN transistors have breakdown fields that are 10× larger than those of silicon. In addition, the on resistance values for GaN HEMTs are typically 10× smaller than those of silicon. These parameters can be used to estimate the reduction in the waste heat generated in power electronics modules when Si IGBTs are replaced with more advanced device technologies. A doubling of the motor voltage from 500 volts to 1,000 volts and reduction of transistor on resistance from a typical 0.200 ohms to 0.020 ohms would reduce Joule heating (where the power converted from electrical energy to thermal energy is equal to the square of the current traveling through an element times the resistance of the element) by a factor of 40. In such a case, rather than generating 1 kilowatt (kW) or more of waste heat, and thus contributing significantly to high inefficiencies and requirements for water cooling, a more advanced power electronics module could generate about 25 watts of waste heat, which could be low enough to allow passive convective air cooling. An additional benefit of GaN HEMTs is their ability to be switched at much higher frequencies than Si IGBTs. This allows significant reductions in the size of passive components such as capacitors, reduces wear on motor bearings, and further improves efficiency. However, these advances have previously been achieved, and it is to such advanced power electronics modules that the presently disclosed concepts are directed.